Resistance variable memory elements, which include chalcogenide-based programmable conductor elements, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. One such device is disclosed, for example, in U.S. Pat. No. 6,849,868 to Campbell, which is incorporated herein by reference.
In a chalcogenide-based programmable conductor memory device, a conductive material, such as silver, is incorporated into a chalcogenide glass. The resistance of the chalcogenide glass can be programmed to stable higher resistance and lower resistance states. An unprogrammed chalcogenide-based programmable conductor memory device is normally in a higher resistance state. A write operation programs the chalcogenide-based programmable conductor memory device to a lower resistance state by applying a voltage potential across the chalcogenide glass. The chalcogenide-based programmable conductor memory device may then be read by applying a voltage pulse of a lesser magnitude than required to program it; the resistance across the memory device is then sensed as higher or lower to define the ON and OFF states.
The programmed lower resistance state of a chalcogenide-based programmable conductor memory device can remain intact for an indefinite period, typically ranging from hours to weeks, after the voltage potentials are removed. The chalcogenide-based programmable conductor memory device can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the device to the lower resistance state. Again, the higher resistance state is maintained in a semi- or non-volatile manner once the voltage potential is removed. In this way, such a device can function as a variable resistance memory having at least two resistance states, which can define two respective logic states, i.e., at least a bit of data.
One exemplary chalcogenide-based programmable conductor memory device uses a germanium selenide (i.e., GexSe100-x) chalcogenide glass as a backbone. The germanium selenide glass has, in the prior art, incorporated silver (Ag) and silver selenide (Ag2Se) for memory function. In chalcogenide-based programmable conductor memory devices such as this example, the two resistance states correspond to the presence or absence of a conductive pathway along a conducting channel established within the chalcogenide glass. Because the electrode in contact with the chalcogenide glass is typically substantially flat, but has a surface roughness, the location and number of conducting channels established within the chalcogenide glass is less than completely predictable and controllable due to the variations in electric field produced by this roughness. If multiple conducting channels are formed, the materials utilized for forming the resistance changing conductive pathway (e.g., the silver selenide and silver) may be divided between the various conducting channels, which could cause diminished or inconsistent memory function with respect to the multiple devices of a memory array. More controllable and predictable formation of conducting channel would be desirable.